When you’re trapped at the bottom of a whirlpool being spun like a sock in a tumble dryer, two thoughts tend to run through your mind. The first, obviously, is: “Am I going to get out of here alive?” The second is more on the lines of: “If I do, I’m never going whitewater rafting again.”

My whirlpool experience was on the Nile in Uganda. But that was two years ago. Now I am standing by the Snake River at the bottom of Hell’s Canyon in Oregon, in the American north-west, waiting for two rafts to be loaded.

So much for near-death pledges. But then who could resist the lure of what is billed as the world’s greatest rafting experience: four days through the deepest gorge on the planet and the promise of mile after mile of Class III and IV rapids? Certainly not my son Tom, whose 21st birthday present this trip is.

Loading the rafts takes the best part of an hour, but then there’s a lot to get aboard: the tents, the kitchen gear, the food, the portable loos - all have to be packed into the supply raft before, finally, the paying passengers are allowed aboard the big 18-footer.

With us are Vernon and Betsy, an Oregon couple for whom the trip is also a kind of celebration. Vern, a warrant officer in the US Army Reserve, has just finished an 18-month tour in Afghanistan and this is his treat before he heads back to the day-job as head of maintenance for the state highways department.

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In the first few minutes of casting off, the rhythm is set for the next four days. Tom and I take the bows, Vern and Betsy the back, and in the middle is the oarsman (there’s no passenger paddling on this expedition).

The river current supplies the motive power, the oars are essentially there for steering, which gives Paul, Morgan or Sam, our three crewmen from Winding Waters River Expeditions, the time to provide a running commentary on the river, the canyon, the wildlife and the history of the place.

Vern, Betsy and Tom fix up their fishing rods and sit happily casting; I lounge back and stare up in awe at the cliffs that rise sheer from the river’s edge. Small-mouth bass are being plucked from the water with almost metronomic regularity, but then we round a bend and the silence of the canyon is broken by the distant thunder of our first set of rapids.

We’ve just got time to stow the rods and find something to hang onto before we’re plunging and bucking on a roller-coaster ride through the white water. It might only be a Class III, but it’s still got quite a kick to it in the shape of a wall of water that bursts across the bows, drenching Tom and myself and liberally soaking the others. The strange thing is, all of us are grinning like idiots as we run out into the still waters on the other side. But that’s the magic of rafting: whirlpools excepted, white water has the transformative power to make a child of the grumpiest of grown-ups.

And so the miles drift by. An hour or so of fishing, of staring up at the mountains, of swapping stories, of watching for wildlife, is followed by five minutes of heart-thumping action. Lunch is taken on a gravel bank under a rock overhang; the crew - with well-rehearsed precision - setting up tables, bringing out food containers from the intricately packed supply raft, and then just as impressively replacing them.

In early afternoon, the two rafts part company. Paul and Morgan take the supply boat downriver to find a campsite for the night while we pull up and trek along an overgrown path in search of a pictograph in a shallow rock overhang. The campsite, when we get there at about 4pm, is on a rare shelf of flattish ground about 20 feet above the river, and about as idyllic a spot as it is possible to imagine.

We pitch the tents under some trees while the crew are busy cooking and the setting sun turns the rock faces all around us into a slide show of mauves and purples. After dinner we crack a beer or two and stargaze for a couple of hours. The night sky is a revelation: the creamy streams of the Milky Way pierced periodically by the sudden streaks of shooting stars and even, once, by a flash from the Space Shuttle as the rays of the far distant sun catch its wings.

By the next morning, we are well settled into the river routine. Drifting, fishing, shooting rapids; mile after mile with barely any trace of man, past or present. For exercise, there are a couple of duckies - inflatable kayaks - to be paddled. White water becomes a completely different experience when every wave is at head height - but the grin factor doubles.

The camping spots prove every bit as perfect as that of the first night. Human presence is so infrequent in some of these places that we become tourist attractions for the local wildlife. Deer come down to sniff at the dinner cooking; a troop of turkeys wanders through the campsite on the way to drink at the river; a six-foot-long bull snake meanders past as we eat breakfast.

Time, by now, has long lost its meaning. As on all real expeditions, we are governed entirely by sunrise, sunset and geography until we reach Dug Bar - some 60 miles downstream from our put-in point - at midday on Day Four.

As we pull into the landing place, a distant speck above the mountains grows into a single-engine Cessna: our transport back to the real world. There’s just enough flat ground here for the plane to land and turn around.

Almost before we can say goodbye to the crew, we’re bouncing down the grass runway. It takes two full circles before the Cessna can gain enough height to clear the canyon, but it gives us the chance for one last look. It was a hell of a good time.

Hell's Canyon basics

Robert Cowan travelled with American Round-Up (01404 881777, www.americanroundup.com), which specialises in ranch and adventure holidays.

He flew in to Portland, Oregon, and drove to Joseph (about three hours) for an overnight stop at Chandler’s Inn, an upmarket B&B, from where he was picked up next morning by Winding Waters River Expeditions for the drive to Hell’s Canyon.

Return flights from London to Portland cost about £840 including taxes and fuel surcharges. Car hire starts from £180 a week for a compact car with unlimited mileage plus all insurances and taxes.

Rafting trips can last from three to six days and cost between $750 and $1,364 (between £376 and £684). All meals are included, but you should take your own beer.

The rafting season runs from June 24 until August 19. Prices start at £415 per adult for a three-day trip, including a first night at a B&B in Joseph, all meals, transport to and from the river, camping equipment, sales tax and the services of experienced guides.